A typical power supply circuit receives power from an AC source. During a normal mode of operation, the power supply circuit generates either a constant voltage or a constant current to a DC load. The power supply circuit includes a first-stage power factor correction circuit that operates to maintain the current drawn from the AC source substantially in phase with the voltage of the AC source. The power factor correction circuit provides a first-stage output voltage to a second-stage DC-to-DC converter circuit. The second-stage DC-to-DC converter circuit generates the constant voltage or the constant current for the DC load.
The typical power supply circuit has a standby mode of operation in which the power supply circuit remains connected to the AC source but does not provide power to a load. The power supply circuit continues to dissipate power even when no power is being provided to the load. For example, the power factor correction circuit includes a resistive feedback circuit from the first-stage output to a feedback input terminal. The resistive feedback circuit dissipates power even when the power supply circuit is in the standby mode. The same resistive feedback circuit is also integral to the operation of an overvoltage detection function of the power factor correction circuit.
The power dissipated by the resistive feedback circuit can be reduced by increasing the resistance of a portion of the resistive feedback circuit; however, increasing the resistance in a conventional manner increases the magnitude of a threshold overvoltage detectable by the overvoltage detection function. Accordingly, increasing the resistance in a conventional manner is not a desirable technique for reducing the power dissipation of the resistive feedback circuit.